Abstract
Measurements of fluorescence were performed to evaluate a wave length shifting epoxy paint for the Fermilab water tank.
Introduction
The water tank being fabricated for Fermilab requires an internal paint to cover its steel walls. Samples of epoxy paint, include a surface layer of wave length shifting paint were tested in a small water tank and reported to yield a significant increase in the observed pulse height.
Samples of a combination of 61-White epoxy paint (TiO) with different thickness of the 69-810-UV were produced by the manufacturer. The available thicknesses of the UV paint were 1, 2, 3, 4 and 5 thousands of an inch.
Measurements were performed with a HP8451A Diode Array Spectrometer from the Particle Detector Group at Fermilab. Illumination was provided by a Mercury lamp with a 313 nm filter, the light being incident at approximately 45 degrees to the surface of the sample. This wave length is a standard for comparing the response of samples to the UV range, specially with scintillators. The diode detector array is located at 90 degrees with respect to the surface of the sample. The illuminated area has a diameter of the order of 0.25".
Results
Each quoted measurement is an average of 5 measurements performed by the spectrometer, no standard deviation is available for the instrument's internal averaging. In addition, measurements were repeated at different spots of the samples. From these later measurements a mean value and a standard deviation are calculated. The number of spots and the number of measurements is indicated for each sample.
The fluorescent response from the samples had a single peak at approximately 460 nm. This peak was integrated between 400 and 600 nm. The illuminating wave length of 313 nm was nearly totally absorbed on the samples and did not produce a diffused reflected peak in the detector. Although no absolute calibration has been performed, the fluorescent peak from these samples is significantly stronger that values obtained with, for example, DMPOPOP as a wave length shifter.
The following table and figure summarizes the results. Reproducibility of the measurements was tested by measuring 5 times the same sample (0.001") on the same spot, listed as the first row in the following table.
69-810-UV # Spots/# Integral 400 R.m.s.
R.m.s. %
thickness Measurements to 600 nm
0.001" 1/5 8943 76.5 0.85
0.001" 4/5 8403 583 6.9
0.002" 1/1 8452
0.003" 2/2 6704 157 2.3
0.004" 4/4 6613 856 12.9
0.005" 4/4 7662 389 5.1
Conclusions
It appears that the thinner the layer of the 69-810-UV paint the largest the emitted light. The variability from spot to spot on a given sample is quite significant and probably indicates a non-uniformity in the application of the paint. The r.m.s. of the repeated measurement is significantly lower than the spot to spot variations within a sample.
Acknowledgments
The assistance of Anna Pla-Dalmau of the Research Division Particle Detector group is gratefully acknowledge.